Magnetic resonance imaging (MRI) relies on the detection of the MR signal from abundant protons in the human body. A radio frequency (RF) receive coil is a device to effectively “pick up” the MR signal from the background of noise for image production. MR signals induced in a RF receive coil are weak signals due to the very small population difference between the two relevant proton energy states at room temperature. One of the challenges in RF coil design is to improve the MR signal detection sensitivity.
One of the approaches to improve signal detection sensitivity and/or field of view is to use multiple receive coils as an array. The basic idea is that instead of making a larger and less sensitive coil that covers the entire volume of interest, plural smaller and more sensitive coils are distributed over the volume of interest. Each individual coil picks up signal and noise from a localized volume. With separate detection circuitry, each coil element receives the image signal simultaneously. Signals from all the coils are finally combined and processed to reconstruct the MR image for the entire volume of interest.
The principle of MRI involves exciting protons and detecting the resulting free induction decay signals. Each proton possesses a tiny magnetic moment precessing about the static magnetic field. The macroscopic behavior of millions of protons can be represented by a resultant magnetization vector aligning with the static magnetic field B0. A strong RF excitation pulse effectively tips the magnetization away from B0. The free induction decay of this magnetization is detected in a plane perpendicular to B0. Thus, for maximal signal induction, the normal direction of a receive coil must be perpendicular to the direction of the static magnetic field B0.
Based on the direction of static magnetic field, commercial MRI systems are either horizontal or vertical. The so-called co-planar type coil arrays have proved to be effective for horizontal MRI systems for the reasons discussed in the previous paragraph. In a co-planar array, surface coils are arranged in a co-planar fashion and distributed over a volume of interest.
In general, such co-planar type surface coil arrays are not very effective for a vertical system because the condition required for maximal signal induction can hardly be fulfilled. Various modifications to the co-planar designs have been proposed with limited success.
It is known that solenoidal type coils have several advantages for a vertical field system, including its sensitivity, uniformity and its natural fit to various body parts. It is advantageous to utilize solenoidal based coil arrays for vertical MRI systems.
To successfully implement a solenoidal coil array, one must be able to isolate solenoidal coils of the array to prevent them from coupling to each other. This is required because all coils in a coil array typically receive signals simultaneously. “Cross-talk” between different coils is undesirable. Thus effective coil isolation is a major challenge in solenoidal coil array design.
A so-called sandwiched solenoidal array coil (SSAC) has been set forth in U.S. patent application Ser. No. 09/408,506. A SSAC consists of two solenoidal receive coils, a counter-rotational solenoidal coil and a second solenoidal coil sandwiched between the two counter-rotational winding sections of the first coil.
The counter-rotational solenoidal coil produces a gradient B1 field that has a double-peak “M” shape sensitivity profile. The second solenoidal coil produces a single-peak profile sandwiched between the two peaks of the “M” shape profile of the first coil.
The sensitivity profile of a SSAC is determined by the summation of an “M” shape double-peak profile and a centralized single-peak profile generated by the two coils. To avoid unwanted dark band artifacts in the array coil sensitivity profile, the geometric parameters of both coils must be set properly. This process is sensitive to the geometries at hand.